TEM study on the healing of crack induced by hydrogen in carbon steel

The microstructures of the crack induced by hydrogen and crack healing have been studied by using the transmission electron microscopy(TEM). These specimens of carbon steel with the hydrogen-induced cracks were cyclically annealed at 1000°C for 10 h. The SEM and TEM observations on the specimens before and after the heat treatment showed that the cracks could be healed completely by heat treatment. The healing of hydrogen-induced cracks was closely related to heat diffusion of Fe and C atoms. The driving force of crack healing resulted from the plastic deformation energy E_s created by the growth of hydrogen bubbles or cracks. The critical condition of healing of bubbles or cracks is E_s ≥2γ/r (where γ is the surface tension, r is the half length of crack). During healing of the hydrogen-induced cracks, the dislocation density around grain boundaries decreased and the dislocation type transformed from tangle dislocation with high energy to vertical and regular dislocation with low energy. The formation of pearlite in distinct layers during healing process indicated the structure of carbon steel recovered too.